A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
Generally, a lithographic apparatus includes a stage configured to hold an object, e.g. a substrate or patterning device, the stage being moveable relative to a reference structure. Positioning of the stage is done using a position measurement system to measure a position of the stage relative to the reference structure in a measurement direction, an actuator system to move the stage relative to the reference structure, and a control system or controller providing drive signals to the actuator system based on an output of the position measurement system.
It is desirable to position the stage in a direction in plane of the object, i.e. perpendicular to the radiation beam imparted to the object, for overlay. Overlay is the accuracy within which layers are printed in relation to layers that have previously been formed, and is an important factor in the yield, i.e. the percentage of correctly manufactured devices.
It is desirable to position the stage in a direction out of plane of the object, i.e. parallel to the radiation beam imparted to the object, for focus of the pattern, for instance onto the substrate, and determines the amount of blur of the pattern on the substrate.
It has been found that the positioning accuracy in one or more directions may not be satisfactory, especially not to meet future demands in lithography.